rcu: Avoid tick_nohz_active checks on NOCBs CPUs

Currently, rcu_prepare_for_idle() checks for tick_nohz_active, even on
individual NOCBs CPUs, unless all CPUs are marked as NOCBs CPUs at build
time.  This check is pointless on NOCBs CPUs because they never have any
callbacks posted, given that all of their callbacks are handed off to the
corresponding rcuo kthread.  There is a check for individually designated
NOCBs CPUs, but it pointelessly follows the check for tick_nohz_active.

This commit therefore moves the check for individually designated NOCBs
CPUs up with the check for CONFIG_RCU_NOCB_CPU_ALL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -1513,7 +1513,8 @@ static void rcu_prepare_for_idle(void)
 	struct rcu_state *rsp;
 	int tne;
 
-	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL))
+	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
+	    rcu_is_nocb_cpu(smp_processor_id()))
 		return;
 
 	/* Handle nohz enablement switches conservatively. */
@@ -1527,10 +1528,6 @@ static void rcu_prepare_for_idle(void)
 	if (!tne)
 		return;
 
-	/* If this is a no-CBs CPU, no callbacks, just return. */
-	if (rcu_is_nocb_cpu(smp_processor_id()))
-		return;
-
 	/*
 	 * If a non-lazy callback arrived at a CPU having only lazy
 	 * callbacks, invoke RCU core for the side-effect of recalculating